In typical multi-engine aircraft configurations, the aircraft engines may be mounted symmetrically on opposing wing structures or on opposing sides of the aircraft fuselage. This symmetrical mounting creates equivalent moment arms for each engine with respect to the vertical axis of the aircraft, which negates any yawing moment induced by any particular engine when both engines are producing equivalent thrust. However, in the event of an engine failure, asymmetric throttle command or some other event that results in one engine producing greater thrust than the opposing engine, several adverse effects may take place.
The primary effect of asymmetric thrust is that the aircraft will tend to yaw in the direction of the engine producing lower thrust because of the greater torque generated about the vertical axis by the engine producing the greater thrust. This effect is often compounded in an engine failure situation where an inoperative engine may produce additional drag while the compressor fan blades create a windmilling effect in response to the incoming airflow. To overcome and control this induced yaw, a counteracting yawing moment may be introduced by deflecting the rudder. When the rudder is deflected, the corrective yawing moment produced by the rudder about the aircraft's vertical axis is dependent upon the velocity of airflow across the rudder, which in turn is dependent on the airspeed. As the aircraft decelerates, the rudder will need to be deflected further to maintain yaw control.
A problem arises, however, when a speed is reached where the yawing moment produced by the fully deflected rudder will just balance the thrust moment. If a roll maneuver is performed at this condition, there is no additional rudder deflection available to prevent the buildup of aircraft sideslip angle. Excessive sideslip angle in a roll may prevent the airplane from rolling at the rate and to the angle that the pilot intended. The amount of adverse sideslip angle may be dependent on the roll rate of the roll maneuver. This may occur primarily during a relatively low-speed rolling maneuver in which the aircraft is rolling toward the operative engine.
In the past, various techniques have been utilized to counter the yawing induced in an aircraft experiencing engine thrust asymmetry, particularly when full rudder deflection is insufficient to maintain control. For instance, one technique to counter yawing was to reduce the engine speed of an aircraft depending on the aircraft's bank angle. However, this solution may only work once the aircraft exceeds a threshold bank angle, even though the pilot may need additional yaw control at bank angles that may be smaller than the threshold bank angle. It also does not take into account the rate at which the airplane reached a given bank angle. Another solution is to design the aircraft with an increased vertical tail size to provide a rudder with sufficient surface area to increase control at slower airspeeds. However, increasing the vertical tail size undesirably increases the aircraft weight and drag, resulting in higher operating costs. Another solution is to increase the aircraft operating speeds so that the slower airspeeds that could result in insufficient rudder control would not be utilized. However, increasing operating speeds would increase the required takeoff and landing distances, thereby limiting the aircraft's airport options. Yet another solution is to reduce the designed aircraft thrust to limit the severity of a thrust asymmetry scenario to a limit that allows for retained controllability using rudder techniques. However, reducing the thrust of the aircraft would require larger takeoff distances and/or limit the payload, thereby also limiting the aircraft's airport options.
It is with respect to these considerations and others that the disclosure made herein is presented.